Solar, Storage

DYE-SENSITIZED CELLS; Dye solar cells – the new kid on the block

DSC modules yearly generated 10-20% more electricity than conventional crystalline-Si modules of the same rated output power

Paul Murray, Andy Thein, Sylvia Tulloch, Hans Desilvestro, Dyesol Ltd, Queanbeyan NSW, Australia

DSC modules yearly generated 10-20% more electricity than conventional crystalline-Si modules of the same rated output power

While dye solar cell (DSC) technology is still a relatively novel photovoltaic (PV) system, it attracts growing interest as a highly credible alternative to standard silicon PV and to the more recently developed thin film technologies. Because of its ability to act as a “light sponge,” DSC offers, compared to traditional silicon technology, superior performance under diffuse light conditions, and when a panel cannot be directed toward the sun at an optimum angle. DSC is the only solar cell technology that offers virtually unlimited choice of coloration, including colored patterns and optical transparency, features that open a large palette of windows, skylight and advertising applications. As an option, flexible DSC panels of low weight can be designed and produced, which is important for powering portable electronics, but also for certain roofing applications where traditional panels are too rigid or too heavy.

Figure 1. a) Schematic diagram of typical glass and/or flexible DSC highlighting the key cell components; and b) various photographs of typical Dyesol glass and flexible DSC devices in various configurations and arrangements.

DSC panels can be manufactured at relatively low cost on production equipment, which is similar to manufacturing lines used by the printing, coating and packaging industries. DSC relies on materials that are readily available in large quantities and relatively non-toxic. Even some of the more precious materials employed in today’s DSC panels, such as platinum and ruthenium, are used in very low quantities in comparison to catalytic converters or computer disks. Moreover, researchers around the world are working on lower-cost alternatives for dyes and other cell components. Compared to all other PV technologies known today, DSC offers the lowest embodied energy, which is very important in a world where fuels and energy will become more and more precious.

This article will showcase some of the advantages of DSC in relation to other existing and emerging PV technologies. State-of-the-art technology will be reviewed in terms of DSC performance, reliability and cost. While first DSC applications will not target markets where minimized cost and grid parity are the main drivers, there are many applications that can exploit the inherent advantages of DSC. Some of these applications will be highlighted.

DSC – marriage between nanotechnology and photosynthesis
Dye solar cells represent unique devices that are based on nanomaterials, where light harvesting and transformation through photochemical and charge transfer reactions show many analogies to photosynthesis. A DSC consists of two electrodes. The first working electrode (WE) substrate (e.g., conductive glass, plastic or foil) is coated with a thin (<20μm) film of nanoparticulate titanium dioxide (T1O2). A very small amount of photoactive dye (a single molecular layer only) such as a ruthenium-based compound is adsorbed to the T1O2 surface. The two electrodes are separated by a sealant gasket and a redox electrolyte layer. The second cell electrode referred to as the counter electrode (CE) consists of a conductive substrate that is coated with a thin layer of a transparent catalyst. Typical glass and flexible DSCs are shown in Fig. 1.

DSC on its way to commercialization
Glass-based DSC modules have been manufactured in small volumes by various companies and by research organizations for many years now. A typical glass module consists of a series electrical configuration and can be readily constructed with minimal materials and low-cost manufacturing equipment. Excellent long-term performance can be achieved with glass based systems due to good barrier properties of glass. Based on 2006 technology, it has been demonstrated, through accelerated light soaking tests, that product life times of more than 30 years in areas of Europe or Canada, and 20 years in large areas of the US, Southern Europe or coastal Australia are feasible [1]. Even better stability can be achieved with the newest technology. A number of companies, including Dyesol and Fujikura, are testing their products and prototypes against international and national standards such as IEC 61646 “Thin-film terrestrial photovoltaic (PV) modules” or Japan Industrial Standard C-8938 [2, 3]. So DSC technology is rapidly evolving toward full commercial viability.

To assess the relative performance, characteristics and differences between various DSCs and panels in comparison to other technologies, we regularly perform many electrical tests.

 

Figure 2. a) Power-voltage curves of metal-based flexible Dyesol DSC at 1 sun illumination for various temperatures; b) power-voltage curves of a crystalline solar cell for various temperatures. The red shaded areas mark the maximum power voltage range between -10 and +70ºC for DSC and from +28 to +80ºC for silicon. The arrow indicates the continuous loss of Vmpp and Pmpp for silicon technology with increasing cell temperature.

Temperature dependence of performance. Figure 2 shows a comparison of power performance curves at different temperatures for Dyesol flexible cells and crystalline silicon cells [4]. The maximum power point voltage Vmpp remains constant and varies by only ±20mV over the temperature range from −10ºC to 70ºC and the maximum power varies relatively little with temperature. In contrast, Vmpp and the maximum power of crystalline silicon cells significantly decrease with increasing temperature. This is, of course, a very undesirable feature of silicon PV, because Si panel efficiency drastically decreases under actual solar conditions in the field compared to 25ºC assessment in the laboratory, where commercial panels are rated. In contrast, DSC performance improves under solar illumination or remains constant, at least up to ~50ºC. Table 1 compares the relative drop of the maximum power point performance Pmpp between DSC and Si for two temperature ranges.

Angular dependence of performance. For any PV technology, it is very important to know how it will perform under real situations in the field, i.e., such as under non-optimal orientation of roofs or façades and under hazy or partially cloudy skies. Comparison involved testing the relative outputs of various panels under a variety of illumination conditions at different angles in order to simulate different orientations of buildings and to investigate performance differences between roof and façade situations. Such data can be used as input for further system design improvements.

Two fixed angles were selected for this experiment, i.e., 35º (=Canberra latitude) to simulate roof-mounted and solar farm type applications; and 90º to simulate building façade applications. The test stand was then rotated along its vertical axis from East to North to West, thereby simulating differently oriented building roofs and walls. Testing involved measuring outdoor current–voltage (I-V) curves for DSC and crystalline silicon panels secured on an adjustable test board. The test board with the mounted panels was rotated in 15º increments and I-V curves were collected for each position.

Typical results obtained from such a test performed close to the Southern autumn equinox are presented in Fig. 3, where angular performance of a Dyesol 6-cell series glass module is compared to a commercially available crystalline silicon device. For both technologies, the power output P for a North-facing orientation 35º from horizontal was taken as the reference, i.e., PDSC,N35 and PSi,N35. The relative performance difference of DSC vs. silicon can then be assessed for each position through the following equation:

 

As can be seen from these results (top curve, Fig. 3), DSC always exhibits an advantage of at least 20% as compared to Si devices when the panels are mounted vertically. This can be attributed to DSC’s stable Vmpp as a function of temperature and light level. As the panels are rotated in the vertical axis toward West, this advantage further enhanced to in excess of 50% for the DSC panel. This is distinct evidence that DSC is capable of generating significantly greater relative amounts of power than some other solar technologies, particularly when the angle of incidence is far from optimum such as on sides of buildings.

In the 35º roof-mounted/solar farm application case, no significant difference in relative output is observed between the DSC and Si devices until the angle relative to the sun exceeds ~45º (NNW or NNE in the specific set-up). As the panels are moved further toward West/90º (or East/-90º) in the vertical axis, DSC devices display a relative performance advantage of up to 30% or greater. Looking closely at the experimental data, in all cases it is observed that the relative short circuit current outputs of both technologies are rather similar.

 Figure 3. Angular dependence for DSC modules vs. crystaline silicon according to the equation. Experimental data from Queanbeyan, NSW, Australia, 03/15/09, only  shown from North (0&deg;) to West (90&deg;).

The maximum power point voltage of DSC devices (and thus fill factor), however, is significantly more stable regardless of the angle of incidence and light intensity. This can be explained by the entirely different principles of operation. Solid-state devices, such as silicon solar cells, are based on charge transport through minority carriers where bulk electron-hole recombination significantly increases at lower light intensities. Due to the photoelectrochemical nature of the DSC system, where charge transport occurs through majority carriers and recombination mainly at the T1O2/dye/electrolyte interface, recombination processes at low light levels have a much lower negative impact on performance. In addition, surface modification and interface engineering of nano-sized T1O2 can further selectively suppress recombination.

Figure 4. Power-voltage curves for Dyesol flexible SDC panels during a range of illumination conditions including 1, 0.3, 0.1, 0.01 sun, and dappled tree shading.

The superior angular dependence of performance for DSC is also manifested through a more stable power output all throughout the day. DSC panels oriented toward the South (Northern hemisphere) or the North (Southern hemisphere) produce significantly more power in the morning and the evening hours compared to a system based on silicon PV of the same nominal rating. This advantage of DSC vs. silicon becomes more pronounced the further the orientation from the horizontal plane deviates from the optimum where the angle from horizontal equals the geographic latitude. Aisin Seiki and Toyota Central R&D Laboratories published a detailed comparison [5] between DSC and crystalline silicon PV in a solar farm-type application and concluded that DSC modules yearly generated 10-20% more electricity than conventional crystalline-Si modules of the same rated output power.

Tolerance to natural shading. Figure 4 shows power vs. voltage curves obtained for a flexible DSC panel. Such a lightweight and durable panel (Fig. 1) can operate in all light conditions. The results show a high charging voltage of >15V is maintained at all light levels ranging from full sun through to darkness. Tests included uniform lighting; dappled shading under natural trees; various orientations toward the sun; and natural hazy and overcast conditions, some of which are shown in Fig. 4. The panel continues to operate in all of these conditions, providing the ability to trickle charge batteries regardless of the light conditions. Additionally, these prototype panels are able to produce up to 7W of power in full sun conditions already, which is very promising.

At which cost?
It has to be pointed out that the generally used metric unit of $/kW can be very misleading. As discussed above, silicon-based PV panels show significantly inferior performance when heated up by the sun compared to a benign laboratory environment at 25ºC. What really counts for any building owner is the cost of a PV installation based on annually produced kWh. In order to seed first markets, our goal is to develop materials with a maximum cost of 70 US$/m2. A detailed analysis indicates that such a goal can be reached with the materials known today [5]. At an anticipated 7% DSC efficiency, this corresponds to $1/Wp. Further cost reductions at the materials level are anticipated for larger-scale manufacturing.

Conclusion
Some of the features of DSC have been examined through various tests in the laboratory and in the field under different scenarios likely to be encountered in practical applications. Advantages include robustness, constant charging voltage output at all light levels, temperature tolerance, improved performance in ‘normal’ solar conditions up to ~60ºC, less angular dependence, camouflage capability, customizable visual characteristics, low energy payback time, low-cost, good demonstrated prototype performance and long-term stability, all which make DSC technology interesting for many applications.

References

 

  1. H. Desilvestro, R. Harikisun, P. Moonie, D. Ball, F. Au, H. Duong, et al., “Long Term Stability of Dye Solar Cells – from Cells to Modules,” 17th Inter. Conf. on Photochemical Conversion and Storage of Solar Energy, Sydney, Australia, 27 July – 1 Aug, 2008. Presentation accessible on http://www.dyesol.com/index.php?element=Desilvestro_IPS-17.
  2. H. Arakawa, T. Yamaguchi, S. Agatsuma, T. Sutou, Y. Koishi, “Study of a Highly Efficient 10 cm × 10 cm Dye-Sensitized Solar Cell,” 23rd European Photovoltaic Solar Energy Conf., Valencia, Spain, 1-5 September, 2008.
  3. T. Kitamura, H. Matsui, K. Okada, T. Yamaguchi, H. Arakawa, “Thermal Durability of Dye-sensitized Solar Cells and Submodules,” 17th Inter. Conf. on Photochemical Conversion and Storage of Solar Energy, Sydney, Australia, 27 July – 1 Aug, 2008.
  4. E. Radziemska, “The Effect of Temperature on the Power Drop in Crystalline Silicon Solar Cells,” Renewable Energy, 28, 1 (2003).
  5. J. Desilvestro, S.M. Tulloch, and G.E. Tulloch, “Volume Manufacture of Dye Solar Cells – A Materials Perspective,” 2008 Inter. Conf. on Nanoscience and Nanotechnology, Melbourne, Australia, 25-29 February, 2008. Presentation accessible on http://www.dyesol.com/index.php?element=Volume+Manufacture+of+Dye+Solar+Cells.

 

Paul Murray received his BS degree with first class honors (chemistry) from the U. of Wollongong, Australia, and is glass project leader at Dyesol Ltd, Queanbeyan, Australia; email [email protected] Also from Dyesol are: Andy Thein, who has a masters of microelectronics engineering from Victoria U., Melbourne, Australia, and is a test engineer; Sylvia Tulloch, who has an MS from the U. of NSW and is the managing director of Dyesol Industries; and Hans Desilvestro, who has a PhD from the Swiss Institute of Technology in Lausanne and is chief scientist.